Send mail as a warning

I would suggest track DeliveryNotification, and give them some time to make reply.
When the time exceeds, send another mail with high priority. This would enable you to confirm delivery of the mail(not sure if the recipient opened or read).

Sample:

//create the mail message
try
{ 
     using(MailMessage mail = new MailMessage())
    {
    //set the addresses
    mail.From = new MailAddress("username@somedomain.com");
    mail.To.Add("recipient@yahoo.com");
    //set the content
    mail.Subject = "This is an email";
    mail.Body = "this is the body content of the email.";
    mail.DeliveryNotificationOptions = DeliveryNotificationOptions.OnSuccess;
    
    //Add "Disposition-Notification-To" for Read receipt
    mail.Headers.Add("Disposition-Notification-To", "<username@somedomain.com>");
     //Set the SMTP server. You can also set the hostname instead of IP Address
    SmtpClient smtp = new SmtpClient("18.204.1.5");    
    
    //Set the user network/domain credentials
    NetworkCredential netCredit = new NetworkCredential("username@somedomain.com", "yourpassword", "DOMAIN");
    
    //Get SMTP to authenticate the credentials
    smtp.Credentials = netCredit;
    
    //Send the e-mail
    smtp.Send(mail);
    }
}
catch (FormatException ex)
{
   //Track  ex.Message
}
catch (SmtpException ex)
{
   //Track  ex.Message
}

Capture/track all exceptions and also track Delivery Failed messages and read reciepts from the inbox set for delivery notification/sender inbox.. Store them in DB/XML etc. Also track the reply for the mail. Now you can easily decide if you need send a warning mail using the data you have captured.

Thanks,

Kuthuparakkal